1. Field of the Invention
This invention relates to a small fan motor such as an ordinary fan motor and a disk drive motor.
2. Description of the Related Art
The fan motor is generally used for cooling or blowing devices. The fan motor is required to be high in efficiency and also low in vibration and noises. Normally, to reduce the vibration and noises, a mold motor is employed for the purpose of increasing the mechanical rigidity of the motor and reducing the transfer of the electromagnetic vibration generated by the motor. JP-A-2001-231192 discloses a mold motor in which the entire stator core including the winding of the motor is molded so that resin is filled between the stacked cores thereby to reduce the transfer of the electromagnetic vibration generated by the stator core.
Another conventional method of reducing the vibration and noises is disclosed in JP-A-8-70550. In this motor, the assembly accuracy between the stator and the bearing unit and between the shaft and the rotor, the accuracy and the assembly accuracy of the parts are improved to suppress the vibration due to the wobbling generated from the error thereby to reduce the vibration and noises of the motor as a whole.
In the conventional motors described above, the unique values of each motor can be changed by molding the motor and increasing the mechanical rigidity of the whole motor. In these conventional motors, however, the resonant point is simply transferred to another frequency band, and the vibration source of the motor is not affected. Therefore, the problem vibration and noises are still generated as far as the relation with the housing for mounting the motor is concerned. With regard to the improved accuracy, on the other hand, the electro-magnetic vibration generated by the motor itself remains equivalent. Therefore, the problem vibration and noises are generated by the resonance achieved in view of the fact that the electromagnetic vibration of the stator has the natural frequency of the system according to the manner in which the housing is mounted.
In recent years, a product set with a fan motor built therein is required to have a highly efficient motor and a reduced size to reduce the power consumption. For this purpose, the efforts are made to improve the winding occupancy rate, reduce the size using the optimum design technique and employ the highly efficient design. An improved motor efficiency is accompanied by a higher output such as a higher torque or a higher rotational speed under the same input conditions. Therefore, the electromagnetic vibration constituting the source of the vibration and noises is correspondingly increased. On the other hand, the product set is required to produce a smaller vibration and noises than the predecessors, and therefore the electromagnetic vibration generated from the motor is required to be reduced.